The present invention relates to an information detecting device for use in reproducing information recorded at high density on a recording medium. More specifically, the present invention relates to an information detecting device for use in reproducing information recorded optically on a recording medium.
High-density file devices of a large capacity have recently been provided on the market such as optical disk devices represented by digital video disks (DVDs) and magnetic disk devices. Signal processing technique capable of reproducing information in a highly reliable manner would be indispensable to realize recording at high density in such file devices. In particular, file devices have been commercialized in succession that uses a partial response maximum likelihood (PRML) technique. The PRML technique is a combination of partial response waveform equalization and maximum likelihood detection. As is well known in the art, this technique corrects a reproduced signal with the waveform equalization and then carries out the maximum likelihood detection in order to best bring out the properties of the maximum likelihood detector, taking reproduction channels into consideration.
Both the optical and magnetic disk devices are suffered from significant intersymbol interference when information recorded thereon at high density is to be reproduced, which decreases reproduction amplitude. This means that a signal noise ratio (SNR) becomes smaller to raise an error rate of detected information. The maximum likelihood detection uses properties of the reproduction channel that has a certain state transition for information detection. It selects time-series patterns having an minimum mean square of errors from all probable time-series patterns that are expected from the properties of the reproduction channel for a four-bit amplitude information series, for example, supplied to the maximum likelihood detector. This allows detection of information at a lower error rate even with a small SNR.
However, the above-mentioned operation is not easy when implemented in a circuit in practice. In this respect, an algorithm called a Viterbi algorithm described in, "IEEE Transaction on Communication, Vol. COM-19, October, 1971, is typically used to select paths recurrently to reproduce information.
Data are recorded on common file devices at the same density for different radial locations in a disk in view of making good use of a limited recording area. More specifically, it is possible to reduce a scale of detection circuit with a detection circuit inherent to the reproduction channel when the PRML detection is performed because the reproduction channel has about the same properties. However, detection capability is significantly deteriorated due to a change in amplitude of an input signal because it complies with fixed channels.
Typical detection techniques add an automatic gain control (AGC) circuit that follow-ups a gain of an amplifier to ensure a constant amplitude of the reproduced signal even when the input signal is fluctuated in amplitude.
However, as will be apparent later, the conventional AGC circuits used for the file device are analogized and are thus not suitable for miniaturization.